Keep Your Cool

Cool roof coatings are gaining attention as a cost-effective and easy way to improve the energy efficiency of buildings. Introduced in the U.S. in the 1980s with the aim of reducing solar energy absorbed by buildings and the buildup of heat gain in urban environments, cool roof coatings have gained even more traction in recent history.

U.S. Department of Energy (DOE) Secretary Steven Chu, Ph.D., has embraced cool roof options as a method to reduce global carbon emissions and slow climate change, and has directed the installation of cool roofs on all DOE facilities. The DOE estimates that cool roofs can reduce energy usage by up to 50% during peak loads and that every 1,000 ft2 of conventional roofing materials replaced with reflective elastomeric roof coatings will offset 10 tons of CO2 emissions annually.

Figure 1. Effect of White Roof Coating on Surface Temperature

Multiple Benefits

One effective method of quickly installing a cool roof is to apply an acrylic elastomeric roof coating over the existing, serviceable substrate. This type of coating can be designed to adhere to a variety of roofing surfaces, providing a way to convert a hot roof into a cool one. Usually white or off-white in color, acrylic elastomeric roof coatings reflect nearly 90% of solar radiation from the roof, preventing that heat from being absorbed into the building. In addition, their high heat emissivity means they readily release any energy absorbed. Acrylic elastomeric roof coatings are most suitable for the horizontal or flat roofs found on many commercial buildings or row homes.

Figure 1 shows the surface temperature over time of a black roofing shingle vs. the same type of shingle coated with a white acrylic elastomeric roof coating. The maximum air temperature reached on the day of the data collection was 90°F. The black asphalt shingle reached a maximum temperature of 160°F at that time, while the same black asphalt shingle coated with a 100% acrylic elastomeric coating stayed below 100°F. The cooler temperatures on the surface of the roof mean lower temperatures inside the building, which reduces the associated load on air conditioning systems as well.

Figure 2 contains data from a room on the top floor of a row home in Philadelphia, PA. The graphs show the difference between the ceiling temperature and the air temperature in that room over four daytime periods. As shown on the left, before the application of an elastomeric roof coating, the peak temperature of the ceiling is typically about 5°F hotter than the room temperature and as high as 9°F hotter. After the coating application, the temperature difference is generally about 1°F and sometimes 0°F, which demonstrates the effectiveness of the coating at reducing heat gain from the roof to the room. It should be noted that some of the temperature variation could also be a result of changes in the air conditioning setting in the room, which would cause room temperatures to drop more quickly than ceiling temperatures and is often small enough to be within the potential measurement error of the sensors.

Applied six to eight times thicker than regular house paint, acrylic elastomeric roof coatings are designed to be durable, yet flexible. Due to their ability to resist water, moisture, frost, salt, ultraviolet (UV) radiation and other outdoor elements, these coatings provide solutions for extending the service life of roofs as well. When an acrylic elastomeric roof coating is applied over a serviceable roof before it fails, the coating can extend the life of that roof for many years. In addition, installing a roof coating can be a cost-effective alternative to roof replacement when the roof is significantly weathered. Using a coating rather than replacing the roof also helps minimize the waste that results from a roof tear-off.

Chemistry Innovations

The benefits of acrylic elastomeric roof coatings are many; acrylic chemistry serves as the key ingredient, enabling the flexibility, adhesion and durability required to drive the performance of the coatings.

Flexible Combination

A roof is constantly expanding and contracting with temperature and humidity fluctuations, the weight of snow and rain loads, wind uplift, and even vibrations of the building. An acrylic elastomeric roof coating must be flexible to accommodate this movement while also tolerating impact from foot traffic, tools and equipment on the roof.

The technology to develop a flexible, yet tough, coating is a challenge. In an acrylic elastomeric roof coating, simply using a soft acrylic monomer to provide flexibility will not work-it will make the coating too tacky, leading to greater dirt pickup and reduced reflectivity. Using a hard monomer and plasticizing it will not work for the same reason. Therefore, the technology design for acrylic elastomeric roof coatings needs to be a unique combination of soft and hard monomers co-reacted with multiple proprietary chemistries that meet the specific needs of the coating.

Many monomer options are available, but consider a basic example of a soft monomer (butyl acrylate) and a hard monomer (methylmethacrylate) combination:

Butyl acrylate (BA) has a glass-transition temperature (Tg) of -54°C, while methylmethacrylate (MMA) has a Tg of +105°C. Because of its low Tg, a homopolymer of poly-BA is extremely soft and can make a fine adhesive. The higher Tg of a homopolymer of poly-MMA means it can be made into hard-coat clear coatings. These monomers can be reacted by emulsion polymerization to create thousands of products.

In the case of acrylic elastomeric roof coatings, joining these two in the right combination can achieve a unique balance of properties, such as greater flexibility with good film toughness. Proprietary chemistries are then added to modify features like chain length, linearity, side-chain functionality, and custom end-use characteristics like surface crosslinking or affinity toward a substrate. In turn, these features can improve properties like dirt pickup resistance, solar reflectivity, water resistance and adhesion to a variety of substrates.

For example, the majority of modified bitumen products on the market are modified with atactic polypropylene (APP). Adhesion to APP-modified bitumen is extremely difficult, but a combination of the right reactants and acrylic polymers can yield an elastomeric roof coating formulation with outstanding adhesion performance on the substrate. Both coatings shown in Figure 3 have been exposed to six weeks of water immersion. The coating on the left is an acrylic elastomeric roof coating designed specifically for adhesion to APP-modified bitumen. It has retained significant adhesion despite the long period of water immersion. The conventional roof coating on the right has blistered and peeled excessively due to ponding water during the exposure.

A more recent development is an acrylic elastomeric roof coating with excellent adhesion to weathered TPO membranes. Like APP-modified bitumen, TPO roofing membranes cause adhesion challenges. The secret to TPO adhesion is reducing the contact angle, or the angle made between the TPO surface and the leading edge of the coating. Figure 4 illustrates why a smaller contact angle is important.

A hydrophilic surface better absorbs moisture and produces a smaller contact angle. The smaller the contact angle, the better the adhesion and wettability. Excellent wettability occurs when the coating “wets out” or perfectly covers the surface. When wettability is poor, the coating pulls or even runs, leaving areas of the TPO uncoated.

Measuring the contact angles of various TPO substrates over time revealed that the angle actually increases as the membrane weathers, unlike most surfaces that decrease in contact angle with weathering. After determining the conditions and materials that were causing the TPO’s increase in contact angle, the angle was manipulated to match that of the roof coating formulation, which ultimately resulted in an acrylic technology with excellent adhesion to weathered TPO.

Figure 5. Effect of Acrylic Elastomeric Roof Coating on Reflectivity

Durability and the Effect of the Sun

A key property required of any roof coating is durability, which implies resistance to the effects of weather and UV radiation degradation from the sun. UV radiation causes a noticeable change in the appearance and durability of many roofing substrates. For example, as the asphalt in asphalt roofing membranes is exposed to solar radiation, the asphalt begins to break down because it is not resistant to UV light. Granules are used in shingles, capsheets, and even on roadways to help protect the asphalt, but UV light still gets through and degrades it. After about six months of UV exposure, a brown chalky residue begins to appear on the surface of an asphalt-based coating and it begins to crack and erode-a clear indication of UV degradation from the sun.

The white pigments in an acrylic elastomeric roof coating are what provide the solar reflectivity. Since the acrylic polymer in the coating is transparent to UV light, solar radiation causes very little degradation of the coating. Thus, white elastomeric roof coatings can provide both high solar reflectivity and outstanding durability.

Efficient reflectivity also prolongs the durability of the roof coating and, ultimately, the roof itself. Acrylic elastomeric roof coatings achieve effective long-term reflectivity due to the ability of the acrylic polymer to resist dirt pick-up and keep the coating white.

Figure 5 shows reflectance data from acrylic elastomeric roof coating exposure testing at a chemical production facility in Philadelphia. This data was collected for eight years-far longer than the three-year durability requirement of ENERGY STAR program ratings. ENERGY STAR requires a coating to be weathered outside for three years and maintain a minimum solar reflectivity of 50, as a value of 50 qualifies a coating as reflective. Even after eight years, these two acrylic elastomeric roof coatings have reflectivity values above the ENERGY STAR three-year requirement.

Technology for a Cool Future

In addition to the benefits they provide, acrylic elastomeric roof coatings are easy to apply and maintain-periodic cleaning and application of a maintenance coat can continually help extend the life of many roofing substrates and improve the energy efficiency of buildings. Durability, adhesion, reflectivity and flexibility remain some of most important characteristics for acrylic elastomeric roof coatings, and advancements in each of these areas will be essential going forward.

Clearly, as cool roofing solutions continue to generate interest and excitement, the need for innovation in the chemistries that drive those coatings will accelerate as well. A continued emphasis on innovation is the only way to fully capture this burgeoning trend and opportunity.

Products

The Handbook of Sealant Technology provides an in-depth examination of sealants, reviewing their historical developments and fundamentals, adhesion theories and properties, and today’s wide range of applications.